This invention relates to production of photovoltaic thin films, such as might be used for solar cells. More specifically, this invention relates to stabilized solutions for the formation of a copper sulfide layer in a photovoltaic device.
Solar cells are capable of converting solar radiation into usable electrical energy. One approach to making solar cells widely being researched at the present time is the production of photovoltaically active thin films of copper sulfide on various semiconductor materials such as cadmium sulfide. The copper sulfide layer is usually less than about 1 micrometer thick, and the semiconductor, cadmium sulfide, layer is usually about 20-50 micrometer thick. When the copper sulfide composition is represented by the formula Cu.sub.x S, it is generally believed that for the best photovoltaic performance x should be above 1.95, and preferably above 1.99 as disclosed in U.S. application Ser. No. 197,414 filed Oct. 16, 1980 and incorporated herein by reference.
In the cadmium sulfide/copper sulfide solar cells, a preferred method of forming the copper sulfide layer is by dipping a layer of cadmium sulfide, which has been deposited on a conductive substrate, into an aqueous acidic solution of cuprous ion. Cadmium ions dissolve in the solution and are replaced in the sulfide crystal lattice by cuprous ions. This type of ion exchange replacement reaction is termed topochemical. Suitable solutions are taught in the previously mentioned application and in U.S. Pat. Nos. 4,086,101 and 4,143,235, both of said patents incorporated herein by reference.
A major problem with the topochemical fabrication of the copper sulfide layer is the presence of cupric ion, Cu.sup.++, in the aqueous curpous ion solution. Cupric ions and cuprous ions compete to replace cadmium ions in the sulfide crystal lattice. The more cupric ions in solution and incorporated in Cu.sub.x S layer, the lower the value of x in the resulting Cu.sub.x S. The cuprous solutions typically used are readily oxidized by air to form cupric ion. Moreover the cuprous compound used to make the solution usually contains traces of cupric ions unless elaborately purified and used immediately after purification. Even in cases where precautions are taken to exclude air, x is often found to be below 1.99 due to random occasional admission of air.
Thus, it would be desirable to find a means of continuously reducing any cupric ion in the cuprous solution to cuprous ion. In that way, laborious purification of the starting cuprous salt would not be necessary, nor would introduction of air cause irreparable damage to the stoichiometry of the solution. However, soluble redox compounds which might perform that function might also attack or interfere with or be occluded by the growing copper sulfide layer. It would be highly desirable to find an agent for continuous reduction of Cu.sup.++ to Cu.sup.+ that would not be soluble and thus would not interfere with the copper sulfide layer and yet would maintain the cupric ion concentration in the solution at a low level.